Carrier aggregation (CA) is a feature in the Long Term Evolution (LTE) standard that allows user terminals to transmit and receive on multiple carriers simultaneously. Carrier aggregation enables operators to use their spectrum more efficiently and increase user data rate and/or throughput. For downlink transmissions in LTE, user data can be transmitted to a user terminal on multiple carriers in the same transmission time interval (TTI), which is 1 ms. Similarly, for uplink transmissions, a user terminal can transmit on multiple carriers simultaneously. Each cell on a given carrier is a serving cell to the user terminal. Each user terminal has only one downlink primary cell (Pcell in short) and one uplink Pcell, but it can have multiple downlink and uplink secondary cells (Scells in short). The Pcells and Scells are user terminal specific, i.e., different user terminals can have different Pcells and Scells. For a user terminal, the Physical Uplink Control Channel (PUCCH) is always associated with the uplink Pcell.
When two downlink serving cells are configured for a user terminal, i.e., one downlink Pcell and one downlink Scell, the user terminal may receive data transmissions on the Physical Downlink Shared Channel (PDSCH) from two downlink serving cells in the same TTI. In this case, the user terminal needs to send Hybrid Automatic Repeat Request (HARQ) feedback to acknowledge transmissions from two serving cells. For carrier aggregation with two downlink cells, up to four transport blocks (TBs) can be transmitted to a user terminal in one TTI (two TBs per carrier). Accordingly, four HARQ feedback bits are needed to acknowledge four TBs.
In LTE, the HARQ feedback can be transmitted on the PUCCH using format 1b with channel selection for two downlink serving cells (one Pcell and one Scell) without cross-carrier scheduling, which allows feedback for up to four TBs. Format 1b is defined in the Third Generation Partnership Project (3GPP) specification TS36.211. In format 1b with channel selection, the HARQ feedback depends not only on the transmitted signal, but also on the PUCCH resource on which the signal is transmitted. To support downlink transmission from two downlink serving cells in transmission modes that support up to two transport blocks, four HARQ resources are needed. Two of the four required HARQ resources are determined based on the index of the first control channel element (CCE) used to transmit the downlink assignment on the Pcell. The other two HARQ resources are selected from among eight resources configured for the user terminal when the downlink Scell is added to the user terminal. The eight resources are divided into four resource sets, each with two resources. Two bits in the downlink assignment for the Scell are used to indicate which resource set among the four configured resource sets should be used to transmit HARQ feedback. Although a user terminal has four HARQ feedback resources, it uses only one resource at one time. The actual HARQ feedback information is carried by not only the signal it transmits, but also the resource it uses. The mapping between the transmitted signal and HARQ feedback resource to HARQ feedback information is specified in the Third Generation Partnership Project (3GPP) specification TS36.213.
The resource allocation approach used in the prior art is sufficient to provide HARQ feedback without resource collision for a maximum of four downlink Scell transmissions per TTI for user terminals with the same uplink Pcell. However, as the number of CA capable user terminals increases, there will be a need to support more than four downlink Scell transmissions per TTI. Additional PUCCH resources have to be added to support more downlink Scell transmissions without resource collision. However, allocating additional resources for HARQ feedback reduces the resources available for transmitting user data.
Thus, there is a need for improved resource allocation methods that minimize the amount of PUCCH resources needed to support more than four Scell transmissions in one TTI for user terminals with the same uplink Pcell while simultaneously maintaining a low probability of resource collision.